Here's the test video, hopefully you can see how exactly its put together, pretty easy.
The cost was $0.00, I had an old skateboard lying around, spent about an hour cleaning the bearings. Concrete bolt found in the garage is the same size for tripod heads. Plenty of PVC piping and wood from past projects.
A 7D mounted on top of the ball head in the video is quite stable with how its assembled and the weight distribution. I can mount the whole thing higher with 2 adjustable stands and super clamps attaching to either end.
It is necessary to get it off the ground since the trucks hang below the rails themselves, the pieces of wood the pvc pipe is on sits on top of 1x1 pieces of wood.
I use two light stands with super clamps to elevate it.
The friction of the bearings on the pvc is so small, I can elevate one side slightly more than the other and have hands free movement from one side to the other.